Institute for Chemical Research, Kyoto University , Uji, Kyoto 611-0011, Japan.
Inorg Chem. 2014 Jan 6;53(1):594-9. doi: 10.1021/ic402740k. Epub 2013 Dec 13.
A crossover from the A-site-ordered double-perovskite structure with Im3̅ cubic symmetry to the simple-perovskite structure with Pnma orthorhombic symmetry is found in LnMn3V4O12 (Ln = La, Nd, Gd, Y, Lu) synthesized under high-pressure conditions. Relatively large Ln(3+) ions (La(3+), Nd(3+), and Gd(3+)) induce the a(+)a(+)a(+) in-phase cooperative tilting of the VO6 octahedra, resulting in the A-site-ordered double-perovskite structure with chemical composition Ln(3+)Mn(2+)3V(3.75+)4O12. Compounds with small Ln(3+) ions like Y(3+) and Lu(3+), on the other hand, crystallize with the Pnma simple-perovskite structure with chemical composition (Ln(3+)1/4Mn(2+)3/4)V(3.75+)O3, where the Ln(3+) and Mn(2+) ions are disordered at the A site. The random distribution of the small A-site cation induces the a(-)b(+)a(-) tilting distortion of the VO6 octahedra. The observed phase crossover is well explained by the structural stability calculation based on the bond-valence-sum model, and the most stable crystal structure gives the smallest unit-cell volume. This A-site-cation size-dependent phase transition between the A-site-ordered double-perovskite and A-site-disordered simple-perovskite structures in LnMn3V4O12 is thus a result of the structural stability due to the cooperative tilting of the VO6 octahedra. The Mn(2+) ions at the A'(A) site contribute local magnetic moments, whereas the V(3.75+) ions at the B site play a role in metallic conduction. The observed magnetic behaviors are consistent with the order-disorder distribution of the Mn(2+) ions at the A site, antiferromagnetism in the A-site-ordered double perovskites, and magnetic spin glass in the A-site-disordered simple perovskites.
在高压条件下合成的 LnMn3V4O12（Ln = La、Nd、Gd、Y、Lu）中，发现 A 位有序双钙钛矿结构具有 Im3̅立方对称到简单钙钛矿结构具有 Pnma 正交对称的交叉。相对较大的 Ln(3+)离子（La(3+)、Nd(3+)和 Gd(3+)）诱导 VO6 八面体的 a(+)a(+)a(+)同相协同倾斜，导致 A 位有序双钙钛矿结构具有化学组成 Ln(3+)Mn(2+)3V(3.75+)4O12。另一方面，具有较小 Ln(3+)离子的化合物，如 Y(3+)和 Lu(3+)，结晶为 Pnma 简单钙钛矿结构，化学组成（Ln(3+)1/4Mn(2+)3/4)V(3.75+)O3，其中 Ln(3+)和 Mn(2+)离子在 A 位无序。小 A 位阳离子的随机分布诱导 VO6 八面体的 a(-)b(+)a(-)倾斜畸变。观察到的相交叉很好地解释了基于键价和模型的结构稳定性计算，最稳定的晶体结构给出了最小的单胞体积。因此，LnMn3V4O12 中 A 位有序双钙钛矿和 A 位无序简单钙钛矿结构之间的 A 位阳离子尺寸相关的相变是由于 VO6 八面体的协同倾斜导致的结构稳定性的结果。A'（A）位的 Mn(2+)离子贡献局部磁矩，而 B 位的 V(3.75+)离子在金属传导中起作用。观察到的磁行为与 A 位 Mn(2+)离子的有序-无序分布一致，A 位有序双钙钛矿中的反铁磁性，以及 A 位无序简单钙钛矿中的磁自旋玻璃。
